make the C part of compiler-builtins opt-out

Author: RalfJung

src/libstd/Cargo.toml

@@ -18,7 +18,7 @@ panic_unwind = { path = "../libpanic_unwind", optional = true }
 panic_abort = { path = "../libpanic_abort" }
 core = { path = "../libcore" }
 libc = { path = "../rustc/libc_shim" }
-compiler_builtins = { path = "../rustc/compiler_builtins_shim", features = ["c"] }
+compiler_builtins = { path = "../rustc/compiler_builtins_shim" }
 profiler_builtins = { path = "../libprofiler_builtins", optional = true }
 unwind = { path = "../libunwind" }
 
@@ -43,9 +43,12 @@ cc = "1.0"
 build_helper = { path = "../build_helper" }
 
 [features]
+default = ["compiler_builtins_c"]
+
 backtrace = []
 panic-unwind = ["panic_unwind"]
 profiler = ["profiler_builtins"]
+compiler_builtins_c = ["compiler_builtins/c"]
 
 # Make panics and failed asserts immediately abort without formatting any message
 panic_immediate_abort = ["core/panic_immediate_abort"]